One way to see this is the continuous building projects constantly surrounding the cathedral from its earliest days, aiming at reinforcing and expanding this impressive site.

Beginning in 1226, city records show that black and white marble blocks were being transported to the cathedral, presumably to be used for the facade and bell tower.

The outer walls, remains of this extension, can now be seen to the south of the Duomo, while the floors of the uncompleted nave now serve as a parking lot and museum.

[8] Built in Tuscan Romanesque style it emphasizes a horizontal unity of the area around the portals at the expense of the vertical bay divisions.

Pisano's work on the lower façade was continued under the direction of Camaino di Crescentino, but a number of changes were made to the original plan.

[2] The changes were probably needed to accommodate the raised nave and di Cecco's more elaborate design scheme, heavily influenced by French Gothic architecture, which caused the apparent division of the upper portion of the cathedral.

The upper portion also features heavy Gothic decoration, a marked contrast to the simple geometric designed common to Tuscan Romanesque architecture.

[5] While most of the sculpture decorating the lower level of the lavish façade was sculpted by Giovanni Pisano and assistant depicting prophets, philosophers and apostles, the more Gothic statuary adorning the upper portion—including the half-length statues of the patriarchs in the niches around the rose window—are works of later, unattributed, sculptors.

[11] Sculpted by Vico Consorti and cast by Enrico Manfrini, the scenes on the door represent the Glorification of the Virgin, Siena's patron saint.

The horizontal molding around the nave and the presbytery contains 172 plaster busts of popes dating from the 15th and 16th centuries starting with St. Peter and ending with Lucius III.

The colonnade in the drum is adorned with images and statues of 42 patriarchs and prophets, painted in 1481 by Guidoccio Cozzarelli and Benvenuto di Giovanni.

The enormous bronze ciborium is the work of Vecchietta (1467–1472, originally commissioned for the church of the Hospital of Santa Maria della Scala, across the square, and brought to the cathedral in 1506).

[5] Against the pillars of the presbytery, there are eight candelabras in the form of angels by Domenico Beccafumi (1548–1550), He also painted the frescoes, representing Saints and Paradise, on the walls in the apse.

Indeed, Giorgio Vasari, an Italian Renaissance painter and architect, claimed the floor was “the most beautiful … great and magnificent pavement ever made.”[13] This undertaking went on from the 14th to the 16th centuries, and about forty artists made their contribution, the majority of whom were Sienese.

The earliest scenes were made by a graffito technique: drilling tiny holes and scratching lines in the marble and filling these with bitumen or mineral pitch.

This technique of marble inlay also evolved during the years, finally resulting in a vigorous contrast of light and dark, giving it an almost modern, impressionistic composition.

[14] The Slaughter of the Innocents by Matteo di Giovanni is one of the most awe-inspiring panels of the cathedral, and has evoked emotional responses from visitors since its creation in 1481.

[14] The majority of the panels in the chancel are grouped together in a large hexagonal portion of the pavement and depict Scenes from the Lives of Elijah and Ahab.

His successor, Paolo di Martino, completed Joshua Defeats the Five Kings of the Amorites and Samson Chastising the Philistines between 1424 and 1426 which contain more details than the previous panels.

[14] Bordering the main altar is a geometric pattern containing five circular panels which depict Mercy and The Four Cardinal Virtues (Fortitude, Justice, Prudence, and Good Government).

[14] The panels date from 1406, as established by a payment made to Marchese d'Adamo and his fellow workers who executed the cartoons of Sienese painters.

[2] The right transept contains five sections of panels: The Seven Ages of Man, Religion and the Theological Virtues, The Story of Jephthah, The Death of Absalom, and Emperor Sigismund.

[14] The cathedral's valuable pieces of art including The Feast of Herod by Donatello, and works by Bernini and the young Michelangelo make it an extraordinary museum of Italian sculpture.

The Annunciation between St. Ansanus and St. Margaret, a masterwork of Gothic painting by Simone Martini and Lippo Memmi, decorated a side altar of the church until 1799, when it was moved to the Uffizi of Florence.

[5] The funeral monument for cardinal Riccardo Petroni (Siena 1250 - Genoa 1314, a jurisconsult of Pope Boniface VIII) was erected between 1317 and 1318 by the Sienese sculptor Tino di Camaino.

[17] The wall tomb of bishop Tommaso Piccolomini del Testa is set above the small door leading to the bell tower.

Duccio's large stained glass window, original to the building, was removed out of precaution during WWII for fear of shattering from bombs or fire.

At the back of the chapel is the Madonna del Voto (by a follower of Guido da Siena, 13th century), that even today is much venerated and receives each year the homages of the contrade.

It was intended as a repository of the books and the manuscript collection of his uncle cardinal Enea Silvio Piccolomini, eventually Pope Pius II.

The walls are covered with frescoes depicting the ten most important scenes from the life of Pope Pius II, to whom the library is dedicated.